A normal bullet is made out of lead and then coverd in a think layer of copper or brass. This type of bullet will do a reasonable job of holding its shape when it hits something, so you get more penetration but less energy transferd to the target, since if the bullet goes through the energy is lost. A hollow point bullet has the tip removed, exposing the soft lead center, imagine a watermelon with the round part cut off, thats what a hollow point looks like. This increases the amount of energy transferd to the person being shot and thus increase the amount of damage done. The bullet will expand and flatten out like a mushroom inside the target. They are technically forbidden by the Hague Convention of 1899 but are the most widly used pistol ammunition by police and civilians in the USA.

A type of bullet that has a lead core and a swagged jacket but leaves the front portion of the projectile exposed. Within the exposed lead is a cavity that can be as much as 2/3 the diameter of the bullet. The purpose of the cavity is to initiate rapid expansion of the projectile on impact there by creating a larger wound cavity,earlier into the penetration depth. As a bullet expands, the wound cavity grows generating more shock and tissue damage. A hollow point projectile can double in diameter if properly designed. For example, a .40 caliber round can become over 3/4 of an inch in diameter after being fired into a "soft" target( I.E. living tissue)